In the fabrication of workpieces wherein high temperature heating is employed to impart such properties to the workpiece as extrudability, malleability and even dimension variation, forge type furnaces are used into which workpieces are inserted for heating and then removed to a separate fabrication station for further processing. Where there are many workpiece insertions and removals it has been found advantageous to provide a relatively thin slot-shaped permanent opening in the wall of the furnace that is adjacent an insertion and removal workstation. The slot-shaped opening is in length close to the full length dimension of the heat area in the furnace. Such a permanent, precisely controlled, opening provides both workpiece insertion and removal efficiency and also a relatively uniform opening into the high temperature region of the furnace providing precision in temperature control.
The furnaces described above have come to be known in the art as slot forge furnaces. These furnaces are large, heavy installations positioned in the throughput chain of the workpiece processing operation such that repairs on an individual furnace may produce major disruptions in productivity. The interior walls of these furnaces are of thick temperature insulating material in block form and, depending on the installation size, the insulating walls are either supported by exterior metal walls or metal bracing. The front is usually considered to be the wall adjacent the workstation. It is shorter than the rear portion by the vertical dimensions of the slot. The front and the rear walls are desirably longer than the front to back dimension to provide more workpiece-processing accommodations. The top of the furnace is arched for strength purposes due to the long unsupported slot. The front or workstation wall is generally an insert positioned under and conforming to the top arch.
A significant refractory or insulating product, which came upon the scene about a decade or so ago, is made of fibrous material and is molded into a variety of shapes. Vacuum formation, sometimes called "vacuum molding", of such fibrous material is currently a well-known technique involving first, the forming of slurry of a material such as alumina-silica fibers in an inorganic solvent or binder. The slurry thus formed is then, in accordance with the technique, placed into a mold and, by means of a vacuum, liquid is drawn off, leaving only the fiber material in a semi-rigid molded shape sometimes called a module. In some cases of special shapes, the molded form is called a log. This insulating material has improved heat insulating properties, is lighter in weight than conventional materials previously employed, and can readily have holes placed in it after fabrication when useful in construction.
While the above described vacuum formed fibrous material has many advantageous properties, it has low resistance to abrasion. This disadvantage has generally precluded its use heretofore because of the damage sustained by the slot-shaped opening in the front wall of the furnace due to wear from the insertion and removal of the workpieces.
As an alternative, a poured front arch wall has been utilized, that is, a wall composed of high-temperature concrete which is shaped to include the requisite slotted opening. However, it has been found that such a fairly high temperature concrete wall is not capable of withstanding the thermal shocks involved when operating at extremely high temperatures in the range of 2,000 to 2,500 degrees F.
The present invention provides a unique solution to the problem of being able to operate at temperatures in the above-noted range; moreover, it enables overcoming the problem of spalling which occurs when a concrete wall that cannot sustain such temperatures is utilized; and of obtaining significant improvement in the lifetime of the front wall, such lifetime being five to ten times that of a concrete wall. The solution results from the recognition that, since the vacuum molded modules can be readily sawn or cut, one opts to fabricate the front wall of vacuum formed modules or logs of judiciously extended length. In other words, one makes an oversized wall. Accordingly, when the slotted opening defined by the wall becomes damaged by chipping or the like as the result of the workpieces constantly being pushed in through the slotted opening such that the opening becomes so enlarged that the proper interior temperature cannot be maintained, the wall is removed, the damaged part is cut away, and the wall is repositioned.